Most nerves in the human body are actually nerve bundles composed of thousands of individual neurons (nerve fibers). These nerve fibers have different sizes designated as A, B, and C fibers, and they carry signals to and from the brain and other parts of the body. The vagus nerve, for example, may have approximately 100,000 fibers of the three different types, each of which carries such signals. Each fiber of that nerve bundle only conducts in one direction in normal circumstances. The A and B fibers are myelinated, that is, they have a myelin sheath in the form of a substance largely composed of fat. The C fibers, however, are unmyelinated.
Myelinated fibers are typically larger, have faster electrical conduction and much lower electrical stimulation thresholds than unmyelinated fibers. Myelinated fibers also exhibit a particular strength-duration curve in response to a specific width and amplitude of stimulation pulse.
The A and B fibers are stimulated by electrical pulses having relatively narrow pulse widths, from 50 to 200 microseconds, for example. The A fibers exhibit slightly faster electrical conductivities than the B fibers, and have slightly lower electrical stimulation thresholds. Compared to A and B fibers, C fibers are relatively much smaller, conduct electrical signals very slowly, and have high stimulation thresholds typically requiring wider pulse widths (e.g., 300-1000 microseconds) and higher amplitudes for activation. Although the A and B fibers may be selectively stimulated without also stimulating the C fibers, the magnitude and width of the pulse required for stimulating the C fibers would also activate A and B fibers.
As noted above, individual fibers of the vagus nerve normally conduct electrical signals in only one direction. Electrical stimulation of the vagus nerve bundle typically activates sufficient fibers to generate neural signals in both directions (bi-directionality). However, selective unidirectional stimulation is achievable through the use of special nerve electrodes and stimulating waveforms.
A neurostimulator device may be used to selectively and electrically modulate a nerve, such as the vagus nerve. The neurostimulator may be entirely implantable, used primarily external to the body with only a small portion of the circuitry implanted, used almost entirely external to the body with only the nerve electrode(s) and associated lead(s) implanted percutaneously. Whether fully or partially implanted, the disorders treatable, controllable or preventable by such nerve stimulation devices include voluntary and involuntary disorders, migraine, epileptic seizure, motor disorders, Parkinson's disease, cerebral palsy, spasticity, chronic nervous illnesses and involuntary movement; pancreatic endocrine disorders including diabetes and hypoglycemia; dementia including cortical, subcortical, multi-infarct, Alzheimer's disease and Pick's disease; sleep disorders including central sleep apnea, insomnia and hypersomnia; eating disorders including anorexia nervosa, bulimia and compulsive overeating; and neuropsychiatric disorders including schizophrenia, depression and borderline personality disorder. One particularly common ENS technique is known as vagus nerve stimulation (VNS), because the stimulation is directed specifically to the vagus nerve. The effect of VNS is to change the polarization of synaptic membranes in nuclei or centers of the brain, resulting in a reduction or cessation of the neuro-psychiatric disorder being treated.
VNS was approved by the Food and Drug Administation (FDA) for therapy in refractory epilepsy in 1997. Although effective for many patients, improvements in the technology are desirable. More particularly, only a portion of refractory epileptic patients cease seizures almost immediately, whereas most patients show a gradual decline in seizure rates for a period of months to years after initiating VNS therapy. In addition, the therapy is either ineffective or only marginally effective for some patients. Also, other illnesses, such as cancer, are not significantly affected by VNS therapy alone.
Magnetic stimulation (MS) techniques are also used to diagnose a number of medical conditions, including specifically neurological and psychiatric disorders. Magnetic stimulation (MS) refers generally to magnetic nerve stimulation (MNS) therapy that is applied specifically to nerve tissue. One particularly well-known clinical diagnostic technique is transcranial magnetic stimulation (TMS). TMS is a technique for stimulating the human brain non-invasively by using the principle of inductance to transmit electrical energy across the scalp and skull of the patient without the pain associated with direct percutaneous electrical stimulation. It involves placing a coil of wire on the scalp and passing a powerful and rapidly changing current through it. This produces a magnetic field which passes unimpeded and relatively painlessly through the tissues of the head. The magnetic field, in turn, induces a weak electrical current in the brain. The strength of the induced current is a function of the rate of change of the magnetic field, which is determined by the rate of change of the current in the coil. In order to induce enough current to depolarize neurons in the brain, the current passed through the stimulating coil must start and stop or reverse its direction within a few hundred microseconds. Additional details on TMS are provided in U.S. Pat. Nos. 6,132,361 and 6,266,556, both hereby incorporated by reference.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to persons of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.